Casting method



April 0, 1951 R. E. CUNNINGHAM 2,548,353

CASTING METHOD Filed Feb. 25, 1949 4 f v 4 V l -13 1 1' I '1 F f I 5 1,1 J 5 7 e :o F]

INVESTOR,

RUSSELL E. CUNNINGHAM Patented Apr. 10, 1951 CASTING METHOD Russell E.Cunningham, Washington, 0., as-

signor to Melpar, Inc., Alexandria, Va.

Application February 25, 1949, Serial No. 78,347

1 Claim. 1

The present invention relates generally to the potting within a castingof resinous plastic material of electrical components generally, andparticularly to the potting of fragile electrical components, such asminiature vacuum tubes or the like, for the purpose of protecting thelatter against mechanical shock, moisture, fungus and various othereffects deleterious to the life and proper functioning thereof.

The invention relates further, broadly, to the potting within a castingof resinous material, of a mechanism having parts which are movableduring operation of the mechanism, and more particularly to the pottingof electrical variable condensers.

Broadly described, the present invention involves potting of frangibleelectrical components, such as electronic tubes, or the like, inpolyester resins, or other resins which are shockproof, rigid, andrelatively non-hydroscopic, but which have a contractibility differentfrom that of the electrical components during cooling or curing of theresins. In general, it has been found that the resinous compoundsutilized in fabricating a resinous casting contract during cooling andcuring, and, in relation to the contraction of glass or metal, contractvolumetrioally to a different extent than the latter. This difference incontractibility may cause mechanical damage to the electricalcomponents, and especially to those which incorporate glass in theirstructures, during casting or potting, and in the case of miniaturevacuum tubes in particular, such mechanical damage results in leakage ofgas into the envelope of the tube, destroying the operability of thetube. In the case of metallic bodies, on the other hand, the contractionof the metal is less than that of the casting, resulting in a defectivecasting rather than in damage to the metallic body, since the castingtends to pull away from the metal.

It is well known, in accordance with prior art practices, to seal vacuumtubes, or other frangible electrical components, within an envelope,container, or covering, prior to potting, the container taking the formof a thin cylinder, and being formed of glass or of a relativelyresilient plastic, in order that the envelope, container, or coveringmay absorb the stresses due to the contraction of the resinous materialduring casting, thus avoiding damage to the components, or defects inthe casting. Various difliculties are found to exist, in practice, inthis procedure, as applied particularly to miniature vacuum tubes.Insertion of a vacuum tube into a container is applied,

in the first place, by considerable possibility of occurred, this is notascertainable until the potting process has been completed, so thatconsiderable economic waste is involved in proceeding according to theknown methods. I have, accordingly, disclosed, in a prior applicationfor Letters Patent of the United States, Serial 13,437, filed August 10,1948, and entitled Potting Electrical Components, a method of pottingwhich involves coating of vacuum tubes, or other electrical components,with a resilient plastic coating prior to casting. This coating may beformulated with standard heat stabilizers and plasticizers and possessesa resilience requisite to enable the coating to absorb the stressesgenerated by the contraction of the main potting compound duringpolymerization and cooling of the latter, and is essentially capable offorming a bond with the resinous casting. The coating may be quicklyapplied, as by dipping, to provide the required protective container.

While the system disclosed in my previous application above referred tois satisfactory for many purposes, it sometimes happens that the totalresilience or give of the resilient coating is insufficient to protectthe fragile component, such as a miniature vacuum tube, especially aftercasting has been completed and during normal operation of the component.It will be realized in this connection that electrical components, suchas vacuum tubes, are frequently equired to operate under extremeconditions of heat or cold, and that in such cases the differentialcontraction of the casting and the components which exist during pottingrecurs during operation of the component. Experience has shown that inextreme cases miniature tubes have cracked after being potted, or whilebeing potted, in accordance with the system disclosed in my priorapplication.

I have, accordingly, devised a further system for potting fragileelectrical components, such as miniature vacuum tubes, by forming aboutthe component, prior to potting, a layer of wax, having a relatively lowmelting point, and which is compatible with the plastic material whichis to form the casting. The casting is then formed about the wax, andafter curing of the casting, the wax removed. In this way an air pocketor chamber is formed between the potted component and the casting, whichforms the most effective cushion conceivable for absorbing differential3 contractions or expansion of the casting and of the component.

Still a further problem which is faced in the art of potting or castingelectrical components, is that of casting a component which comprisesmechanical parts which must be moved during operation of the component,at least intermittent-.

1y. Examples of components of this character are: variable condensers,small electric motors, variable inductances, potentiometers, variableresistances, and the like. I have elected to disclose my invention asapplied to the pottingof component having movable mechanical parts, andspecifically, a variable condenser. It is to be understood, however,that the principles of the invention may be applied to various othertypes of components, and that the disclosure as applied to variablecondensers is intended to be non-limiting, and for the purpose ofproviding a specific example only.

The process of potting an electrical condenser in accordance with thepresent invention involves the steps of initially potting the condenserin a casting of wax, leaving the rotor shaft of the condenser uncovered.After the wax has solidified, the wax casting may be further cast orpotted in a casting of resinous plastic material, which may be providedwith an aperture suitable for removing the wax. After the plasticmaterial has cooled and polymerized the entire casting may be reheatedfor the purpose of melting the wax, and the latter may then be permittedto escape via the aperture above referred to.

Upon removal of the wax from the outer casting there remains a spacewithin the outer casting, which is partially occupied by the variablecondenser, and, if the wax originally covered completely all the platesof the condenser, the movable plates thereof will be free to moverelative to the stationary plates thereof. Further, the wax casting maybe so designed originally as to leave free some portion of the condenserand particularly some portion of the base or supports thereof. Thesethen, come into contact with the plastic materialduring the potting ofthe outer casting, and form a bond therewith, which subsists afterremoval of the wax, so that the finished casting forms a support forrigidly securing the condenser.

Obviously rotation of the rotor shaft must be provided for in thearrangement described. This maybe accomplished in two distinct ways, oneof which involves merely the provision of an aperture in the resinouscasting through which the shaft may protrude, and the other of whichinvolves fitting of a rotary seal or rotary joint about the shaft of thecondenser prior to casting thereof in the resinous material. Theresinous material is then cast about the seal at its outer diameter, andestablishes there a tight joint. Utilization of the rotary seal asdescribed provides a casing for the condenser which is airtight andmoistureproof, while elimination of the seal provides merely a casingwhich is valuable for protecting the condenser against mechanical shockor other mechanically induced damage.

Proper choice must be made of the composition of the wax material whichis utilized in the potting process, so that polymerization of thepotting compound is not inhibited at the inter-face between the maincasting and the wax, which serves to reduce stresses at or adjacent tothe inter-face and results in a smooth and uniform casting. If a wax isused which is non-compatible with the resinous compound forming the main4 casting, polymerization of the latter adjacent to the wax will beprevented over a relatively wide volume of resinous material, and thecasting will be unsatisfactory.

It is, accordingly, a broad object of the present invention to provide anovel method for potting fragile objects within a mass of shockproof,rigid and relatively non-hydroscopic material, such as polyester resins,or the like, making provision for an air space between the body of thecastin and the objects themselves.

It is a more specific object of the present invention to provide amethod of potting electronic vacuum tubes, or other fragile electricalcomponents,- in polyester resins or other casting materials, in a mannerwhich will provide for maximum relative expansion of the casting and thevacuum tube or other component without mutual damage or mechanicalcontact.

It is more specifically an object of the present invention to provide,as a new article of manufacture, electrical components in potted form,cast within a mass of resinous material, which forms a casing for thecomponents, separated by an air space from selected portions of thecomponents. I

The novel features which are characteristic of my invention are setforth 'in the appended claims. The invention itself, however, will bestbe understood'by reference o'fthe ronowm 'specification, when consideredin conjunction with the accompanying drawings, in which:

Figure 1 illustrates in traverse section, avariable radio condenser,potted in wax preliminary to potting of the wax in resinous castingcompound;

Figure 2 is a sectional View Corresponding with Figure 1, whereinthe'cast'in'g compound is shown encasing the wax casting;

Figure 3 illustrates in seetional view the s'tru'c ture of Figure 2 withthe wax removed to provide a finished product;

Figure 4 illustrates in sections. miniature vacuum "tube coated with waxpreliminary to casting thereof in resinous casting compound;

Figure 5 illustrates the structure of Figure '4 of the drawings encasedin r'esinous'casting compound, prior to removal of 'the wax from the"interior of the casting; and,

'Figure 6 illustrates a pottedv'acuum tube as manufactured in accordancewith the methods of the present invention. 7

Referring now specifically to -Figure l ofth'e accompanying drawings,there is illustrated a variable condenser i of generally conventionscharacter, having fixed plates 2 and variable plates 3, which aremutually inter-meshed in the usual fashion. The variable plates 3 aremounted on and secured to a shaftli,forrotationthereby with respect tothe fixed plates 2. The shaft 4 rotates in bearings provided in bracketarms 5 for that purpose, and the bracket arms themselves are secured toand supported by 'a base member t, which further supports one or-moreflanges 1, extending parallel to the shaft 4, and which are utilized forsupporting and securing the stationary plates- 2 In accordance with thepresentinvention, and as 'an'initial step looking to the potting of thecondenser within resinous casting compound, the

condenser is cast or potted in a solid block of wax 8, by dipping thecondensers into the wax when in a molten state, and then .ipermittingthe wax to solidify. While substantially thegentire condenser is coatedor covered with wax during this step of the process, it will be clearfrom the drawings that the shaft 4 is permitted to protrude from the waxcasting 8, for purposes which will appear hereinafter, and further thatthe base member 6 remains uncovered at least in part by the wax of thewax casting 8.

After the wax casting 8 has solidified, a rotary seal 9 is mounted onthe shaft 4, in intimate contact with the wax, the seal 9 being anysuitable such device which is readily commercially available, and whichis capable of permitting free rotation of the shaft 4 therein whilesealing the shaft against passage of moisture or gases at the junctionof the seal and the shaft.

Thereafter the casting 8, and also the seal 9, are dipped Within liquidresinous casting material contained in a suitable form, the clippingprocess being accomplished with the shaft 4 in a vertical orientation,and the surface of the resinous casting compound being arranged parallelto the wall ii) of the wax casting 8, and at a slightly higher levelthan the Wall IU of the wax casting 8, so that the resinous castingmaterial at least partially covers the outer diameter of the seal 9, butdoes not attain to or enclose any portion of the shaft 4 as it extendsbeyond the seal 9. The form containing the resinous casting material isprovided, moreover, with a projection designed to reach from the bottomof the form to the wall H of the wax casting 8, which may, for thispurpose, rest on the projection. Accordingly, when the resinous compoundhas been permitted to cure and polymerize to form a casting I2, anaperture 13 is provided in the wall H of the resinous casting, whichotherwise completely encloses the wax casting 8, and seals the externaldiameter of the rotary seal 9.

After the main resinous casting l2 has cured and polymerized, it may beheated to a temperature sufficient to melt completely the enclosed waxcasting, the wax being then permitted to escape via the aperture l3.After all wax has been removed, the aperture [3 may be plugged by aplastic plug I4, whereupon the process of enclosing the condenser iscomplete.

It will be noted that by following the steps above described, a productresults which consists of a variable condenser having movable plates 3,which are free to move with respect to the stationary plates 2, byrotation of the shaft 4, the variable condenser being, however,completely enclosed by the resinous casting material, and, accordingly,protected against mechanical shock or .damage of any type, and furtherisolated from the external atmosphere by seal 9, and, accordingly,maintained free of fungus, moisture, and the like.

It will be clear from the process described and from the description andillustration of the resulting product that the process may be applied toother structures than variable condensers, and particularly to electricmotors, potentiometers, variable inductances, and the like, whichcomprise mechanically moving parts and a rotatable shaft, and whetherthat shaft is used for providing output power, or for transferring powerto the mechanism internally of the resinous casting.

A modification of the present invention involves the casting ofrelatively fragile objects, which do not comprise mechanically movableparts, but which it is desired to protect against shock or mechanicalimpact. One obvious object of this character is a vacuum tube having aglass envelope. While vacuum tubes have been potted in accordance withprocedures now understood by those skilled in the art, and particularlyin ac cordance with a method illustrated and .described in my priorapplication, Serial 13,437, filed August 10, 1948, to which referencehas been made hereinabove, these methods all have the inherent defectthat the tube is essentially in contact with the resinous casting,despite the fact that the contact may be via a relatively flexible orsoft protective layer. It is a primary virtue of the present inventionthat the protective layer may be dispensed with entirely, which resultsin optimum provision for differential expansion or contraction asbetween the tube and its resinous casting, and, further, in that thespace provided between the casting and the tube may be made of an.desired dimensions with extreme simplicity and rapidity.

Referring now more specifically to Figure 4 of the drawings, I haveillustrated a miniature Vacuum tube 20 having a glass envelope 2 I, anda base 22, from which protrude leads 23 which are utilized forenergizing the tube and deriving signal therefrom. It will become clear,as the description proceeds, that while I have chosen to illustrate avacuum tube 20 which is provided with a base 22, that provision of thebase is not essential to the present invention and that vacuum tubes ofany type may be utilized in the practice thereof, and especially vacuumtubes having no bases, but which consist essentially of glass envelopesfrom which protrude the requisite leads.

The glass envelope 2|, may, in accordance with the present invention, becoated with wax 24 to any desired thickness, either by a casting processor by a simple dipping process. After the coated wax has solidified, thewax coated tube 20 may be enclosed in resinous compound, preferably inliquid form, and preferably by a molding process, the mold beingprovided with a projection designed to introduce an aperture 25 in thefinished casting. The resultant casting 26 may be of any desired shapeor size provided only that it covers the wax envelope 24, and at least aportion of the base 22 of the vacuum tube 20, in the event the latter isprovided with a base, or encloses the entire tube in the event the tubehas no base, but in any event permitting protrusion of the requisiteleads so that the vacuum tube may be connected in circuit. After theresinous casting has cured and polymerized, the wax 24 may be heated anddrawn 01f Via the aperture 24, leaving free space between the tubeenvelope and the casting. Thereafter the aperture 25 may be plugged upin any convenient manner, as by a plug 2?, and the structure is readyfor use.

While I have described and illustrated two specific embodiments of myinvention, it will be clear that variations of structure and method maybe concerned by those slnlled in the art without departing from the truespirit and scope of my invention.

What I claim and desire to secure by Letters Patent of the United Statesis:

A method of potting an electronic tube having a frangible envelope and anon-frangible support, comprising, dipping only said envelope inliquified wax, removing said envelope from said liquid wax, hardeningsaid wax, enclosing said wax coated frangible element totally and saidnon-frangible support partially in a hot resinous casting compound,cooling and solidifying said resinous casting compound to provide acasting surrounding said wax coated frangible envelope totally and saidnon-frangible support partially, and removing gum-ass to leave -.a. sealbetween gaid Lsuppoltt and said \casting.

RUSSELL AREEERENCES CITED The following :references are of record insthe file of this patent; 7

:Name 113%? liipharsi W -Feb- 1 1903

